Much of the research on homologous recombination has been focused on the bacterial RecA protein. The eukaryotic Rad51 protein forms very similar nucleoprotein filaments to those formed by RecA. Rad51 is essential in higher organisms, including humans, and is believed to play a large role in the maintenance of genome stability. Electron microscopic (EM) studies of these protein-DNA filaments can yield important information about mechanism in these highly conserved structures. Significant improvements in resolution are now possible using cryo-EM of frozen-hydrated specimens, as well as new computational approaches. These studies are beginning to show how domains are arranged in these filaments, the conformational changes that are associated with activation of the filaments, as well as revealing interactions between these filaments and other proteins. Helicases have the same nucleotide-binding core present in the RecA and Rad5 I proteins. It has become apparent that a large number of helicases involved in DNA recombination, replication, repair and transcription form hexameric rings around DNA. Mutations in helicase genes have been shown in humans to lead to xeroderma pigrnentosa, Cockayne's syndrome, Werner's syndrome and Bloom's syndrome. There are numerous suggestions that many of these proteins may not actually be helicases, but motor proteins with diverse functions. In fact, more than 2 percent of human genes may encode for "helicases." EM and single particle image analysis will be used on different hexameric replicative helicases, including E. coli DnaB, Simian Virus 40 large T, archaeal MCM and bacteriophage T7 gp4, to study the interaction with DNA, conformational changes that occur during the ATPase cycle, and cooperative interactions among subunits. These studies will be done in collaboration with crystallographic efforts, and high-resolution data on subunits and domains will be combined with lower resolution data on quatemary organization. The overall question that will be addressed is whether the conserved nucleotide-binding core present in RecA, Rad5 1 and the helicase superfamily has led to a conservation of mechanism across this huge class of proteins. Structural studies are now revealing that other proteins active in recombination and repair, with no apparent homology to this class, also form rings. Examples are translin, involved in chromosomal translocation and RNA processing, and Rad52. Studies of these proteins will be conducted in parallel, with the aim of understanding whether convergent mechanisms of protein-mediated recombination and repair exist.